1. Field of the Invention
This invention relates to a brushless direct current electric motor and more particularly to such motor in which smooth rotational operation may be obtained through elimination of cogging torque otherwise caused during rotor rotation.
2. Related Art Statement
Various constructions have been proposed and adopted in this type of brushless direct current electric motors. Although related publications or literature are not specified herein, the construction commonly used in the prior-art is that in which the pitch of the stator teeth is related to the magnetic pole pitch of the magnetic pole section of the rotor in the manner shown in FIGS. 8 to 11.
FIG. 8 shows in a developed plan view the relation between a magnetic pole section 3 of a rotor, not shown, and the stator teeth 2 of the stator 1 employed in a brushless direct current electric motor. It is seen that the center O.sub.1 of a given stator tooth 2 and the center O.sub.2 of the magnetic pole part 3 are offset by an angle .theta. relative to the tooth pitch Ps of the stator teeth 2.
In general, when the pole part 3 is rotated in the above described state, the cogging torque T fluctuates in the direction of the angle .theta. and occurs continually and repeatedly at the tooth pitch Ps or the interval between the adjacent stator teeth 2.
The construction shown in FIG. 10 represents a typical construction of the prior art in which the tooth pitch Ps of the stator teeth 2 provided on the stator 1 is made equal to 20.degree. C.
The respective magnetic pole parts 3 are formed equiangularly on the periphery of the rotor 4. The structure is of six-pole construction with the magnetic pole pitch Pr made equal to 60.degree..
Thus, when the rotor 4 of the above described prior-art construction is caused to rotate, cogging torque occurs every where in phase so that cogging, several times as large as that shown in FIG. 9, is produced. In short, the magnitude of the produced torque corresponds to that shown in FIG. 9 multiplied by the number of magnetic pole parts 3.
With the above described construction of a conventional brushless direct current electric motor, the amount of cogging torque produced increases in proportion to the increase in the number of rotor poles so that smooth rotation is not achieved when the motor is used for driving control equipment or a VTR.